Phosphoric acid has been used as an additive in lead–acid battery electrolytes, but not as a replacement for sulphuric acid.
When added in small amounts, phosphoric acid forms stable compounds on the positive plate that help reduce corrosion, shedding of active material, and sulphation. This effect can noticeably extend the cycle life of a battery, especially under deep-cycle use where the plates are stressed over many charge/discharge events.
The benefits:
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Extended cycle life in renewable energy and stationary applications.
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Reduced sulphation, which is one of the main failure modes of deep-cycle lead–acid batteries.
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Improved adhesion of active material to the grid, reducing plate degradation.
The trade-offs:
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Higher internal resistance reduces cranking performance and fast discharge capability, making it less suited for starter batteries.
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Reduced charge acceptance compared to conventional sulphuric acid electrolyte.
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Primarily beneficial in deep-cycle applications (telecom, UPS float, renewable storage), rather than automotive cranking or TPPL AGM designs like Odyssey.
Industry use
Some tubular flooded cells and stationary batteries have adopted phosphoric acid additives, but it’s less common in sealed AGM or Gel batteries. Those chemistries are already optimised for long life, and TPPL (Thin Plate Pure Lead) designs such as Odyssey focus on high cranking output rather than maximising cycle life.
👉 In short: Yes, phosphoric acid can improve cycle life, but it does so at the expense of high-current performance. That’s why it’s sometimes used in deep-cycle stationary batteries, but not in starting or TPPL AGM batteries.